It is well recognized that in the preparation of the thermosettable ethylenically unsaturated epoxy ester compositions that a polymerization inhibitor such as hydroquinone or phenothiazine must be added and that air or oxygen must be bled into the reactor in order to keep the polymerization inhibitor active. In the absence of oxygen, the reaction mixtures polymerize inhibitor quickly to an unusable mass. More recently, it has become known that the shelf life or stability of ethylenically unsaturated epoxy ester compositions containing a conventional polymerization inhibitor such as hydroquinone, methyl hydroquinone or phenothiazine is also dependent upon the presence of oxygen in the resin composition to keep the polymerization inhibitor active. The storage stability problem is most noticeable when resin compositions are shipped during periods of high outdoor temperatures and also when the resin compositions are stored for prolonged periods of time. U.S. Pat. No. 4,129,609 teaches that the storage stability of these resin compositions is improved by employing as the polymerization inhibitor a thiuram compound containing a group expressed by the structural formula: ##STR1##
The use of dinitrophenols, tertiary butyl catechol and alkyl hydroquinones as inhibitors of free radical polymerization which enhance the storage stability of unsaturated polyester resins and unsaturated epoxy ester resins is taught in U.S. Pat. No. 4,083,890.
Japanese Patent Publication No. 52-108478/1977 is directed to unsaturated epoxy ester resin compositions to which hydroquinone and a mononitrophenol have been added to control the tendency to generate heat while forming a gel during polymerization. Mononitrophenols employed in the examples are o-nitrophenol, 2-nitro-p-cresol, and 4-nitrocatechol.
It would be desirable to have anaerobically effective polymerization inhibitors which greatly extend the shelf life without adverse effects on the curability of the resin compositions. U.S. Pat. No. 4,129,609 teaches that nitro compounds are not anaerobically effective polymerization inhibitors regardless of the amount employed. Anaerobically effective polymerization inhibitors are those inhibitors that are effective at very low levels of oxygen down to the absence of oxygen.